Device for defining a horizontal plane

ABSTRACT

A device for defining a plane to determine relative elevations comprising in combination a stationary housing (7) having an aperture (8) and locating a pair of vertical reflecting surfaces (6) at right angles to each other which may be a pair of mirrors (5) or a right angle prism (32) and a levelling rod or staff (14) adapted to be positioned at a vewing distance from the housing (7) and incorporating a light source (12) and an eyepiece (15) whereby an observer at the location of the levelling rod (14) can move the light source (12) up and down the levelling rod (14) until a reflection of the light from the light source (12) is seen in the pair of reflecting sufaces (6). A reading can then be taken on the levelling rod (14) at a point midway between the eyepiece (15) and the light source (12) which defines a horizontal line from the reflecting surfaces (6).

FIELD OF THE INVENTION

This invention relates to a device for defining a plane, in particular,but not exclusively, a horizontal plane, such as would be required incivil engineering construction work.

BACKGROUND OF THE INVENTION

The need to find the relative elevations of points is extremely common,especially in construction work. This need is usually met by using asurveyor's level and levelling rod, or sometimes by a laser level and asensor on a rod.

These methods are quite effective but in the case of a surveyor's leveltwo people are required, one at the level and the other at the graduatedlevelling rod.

A laser level utilises a beam to form a horizontal plane and can be usedby one person but it is quite expensive.

An object of the present invention is to provide a simple inexpensivedevice to enable the required elevation differences to be easilydetermined by one person.

The concept of the present invention is that a pair of verticalreflecting surfaces at right angles to each other, such as a pair ofvertical mirrors at right angles to each other, or a right angled prismwill reflect light back to its source, provided that the light source isnot above or below the reflecting surfaces nor outside the field of viewof approximately 90°.

If the mirrors have a horizontal mark on them, the horizontal planecontaining that mark can be found by moving the observer's eye up anddown until a reflection of the observer's eye coincides with the mark.This principle is already known, for instance in Australian Patent No.539,881 (67092/81) where a telescope is used to assist in aligning thereflected image with the mark on the mirrors.

However, the device described in the abovementioned patent has a numberof defects which make its use in practice difficult.

One defect is that it is often difficult to see the reflection of theobserver's eye or telescope. One reason for this is that the reflectedimage of the observer's telescope is narrowed as the observer approachesthe limit of the field of view near a vertical plane containing amirror. The width of the reflected image of the telescope tends towardszero as this limit is approached. A second reason is that undesiredreflections of adjacent walls and the like appear in the mirrors andmakes it hard to see the desired reflection. This is compounded if theobserver and his telescope are in subdued light and the walls or otherobjects are brightly illuminated.

The second problem is that it is difficult to maintain the mirrors inaccurate vertical planes because uneven thermal expansion of thesupporting tripod and frame upsets the vertical alignment. This is ofparticular concern because the observer will normally be at thelevelling rod rather than the mirror device and hence may be unaware ofthe mis-levelment.

The present invention eliminates or greatly reduces the above problems.

Firstly, the difficulty of seeing the required reflection is reduced byusing a bright light source at the observer's location. This light ispositioned vertically above or below the position of the observer's eye.The observer moves his eye up and down until the reflection of the lightis seen in the mirrors. The horizontal plane passing through the mirroris then halfway between the light and the observer's eye.

The advantage of using a light is that its reflection can easily be seeneven if its subtends a very small angle at the observer's eye. Thisallows very small mirrors or a small prism to be used, each beingtypically 1 or 2 mm high and 10 or 20 mm wide. If the vertical apertureof the light is also small, say 2 or 4 mm, then it is possible toresolve elevation differences of a millimeter or so which is ample formost construction work.

Being able to use small mirrors or a small prism is not only economicalbut it also facilitates using an automatic levelling device to maintainthem in a vertical plane.

The ease with which a light can be seen is dependent on the darkness ofits background.

To facilitate seeing the reflected light the mirrors may be located in ahousing which is very dark in color and which has slats around it tokeep direct sunlight from illuminating the exterior of the container.Each pair of slats acts to make a heavily shaded area such as anobserver would see looking into the interior of a black box. The slatsalso help to keep the interior cool.

The overall effect is that the housing appears to the observer as anextremely dark object. The reflected light shining out from inside canthus be easily seen, even when the observer is approaching the limit ofthe field of view. Another function of the housing is to shield unwantedreflections of brightly lit adjacent walls and the like which may bemistaken for the desired reflection or the light at the observer'slocation. A further precaution which may be taken to enable thereflection of the light easy to see is to provide a dark colored areaimmediately surrounding the light source and facing the mirrors.

The second problem to be overcome is to maintain the mirrors in accuratevertical planes regardless of thermal expansion over time.

In an embodiment of the present invention this is achieved by attachingthe mirrors to a pendulum which has a damping arrangement to bring it torest within a reasonable time.

The pendulum may be suspended by a system of threads, tapes or the likedesigned to allow it to oscillate freely in any direction withoutrotating.

The third problem with the device in the above-mentioned patent is thatthe eye of the observer has to be at the same elevation as the levellingdevice and that this may not be convenient on a sloping site. Thisdifficulty is overcome in an embodiment of the present invention by thefollowing means.

Instead of the observer being required to move his eye up and down thelevelling rod to find the reflection of the light and hence the desiredhorizontal plane, he can keep his eye at the convenient height and movethe light source up and down the rod until a reflection of it is seen atthe mirror device.

The principle underlying this is that when the light and the eye areseparated vertically, the horizontal plan passing through the mirrorsand perpendicular to them is always half way between the light and theobserver's eye. The reading on the levelling rod corresponding to thehalf-way point can be read directly using a special scale for thispurpose.

SUMMARY OF THE INVENTION

According to the invention there is provided a device for defining aplane to determine relative elevations comprising in combination astationary apertured housing part locating a pair of vertical reflectingsurfaces at right angles to each other and a levelling rod adapted to bepositioned at a viewing distance from said housing part andincorporating a light source and an observation sight which allows thedesired plane to be found by observing the reflection of the light insaid pair of reflecting surfaces.

In order that the invention and its manner of performance may be morefully understood, reference will now be made to embodiments of theinvention as illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a part-sectional side elevation of a mirror housing of anembodiment of the invention;

FIG. 2 is a part-sectional plan view taken along the line 2--2 of FIG.1;

FIG. 3 is a plan view of the right-angled mirror reflecting deviceillustrating the reflection of light from the pair of mirrors;

FIG. 4 is a side elevation illustrating the mode of operation of theinvention;

FIG. 5 is an end elevation of the levelling rod of FIG. 4 lookingtowards the mirror housing;

FIG. 6 is a diagrammatic elevation illustrating the use of the inventionon sloping ground;

FIG. 7 is a part-sectional side elevation similar to FIG. 1 illustratingthe use of a prism as the reflecting device with a modified suspensionsystem;

FIG. 8 is a part-sectional plan view taken along the line 8--8 of FIG.7;

FIG. 9 is a side elevation of the prism of the embodiment of FIGS. 7 and8 taken through its axis of symmetry; and

FIG. 10 is a diagrammatic plan view of a mirror or prism housing with afield of view of 360°.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 6 of the drawings which illustrate an embodimentof the invention utilizing a right angle pair of mirrors 5 as thereflecting surfaces. The mirrors 5 are referred to hereafter as themirror pair 6. The mirror pair 6 is mounted in a housing or container 7which is preferably, but not exclusively, of circular configuration. Thehousing 7 has an opening 8 which defines the field of view ofapproximately a right angle subtended at the intersection of the mirrors5. Slats 9, and vertical ribs 10 are provided to shade the exterior ofthe housing 7 from the direct rays of the sun and hence make the housingappear to the observer as a dark object. They also serve to keep thehousing cool. The slats 9 and the interior and exterior of the containerare dark in color, preferably matte black.

In FIG. 2, the observer's eye is shown at 11 and in plan this alsocoincides with the light source 12 carried by a fitting 13 slidablymounted on a levelling rod 14, the light source 12 being alwaysvertically above or below an eyepiece 15 on fitting 13. The lighttravels along the ray 16 and is reflected directly back into theobserver's eye when the eye is at the correct elevation.

The path 17 represents a potential undesired reflection from a wall 18which may be highly reflective. The reflection from wall 18 wouldconfuse the observer were it not blocked out by the housing 7. Entry ofwind, dust and water into housing 7 is prevented by a glass window 19.

FIG. 1 illustrates the mirror pair embodiment in elevation. The mirrorpair 6 is attached to a pendulum arrangement 20. The pendulum 20 pivotsat the point 21 where it is attached to a support 22 which is attachedto the housing 7. The pendulum supporting system consists of groups oftwo or more threads 23 each attached top and bottom in turn to a plateor disc 24. Each group of threads is in a vertical plane perpendicularto the one below it. With at least two groups of threads the pendulum 20is free to oscillate as a pendulum in any plane without rotating aboutits vertical axis.

Although other pivoting arrangements could be used for the pendulum, aparticular advantage of the arrangement shown is that it operateseffectively in any vertical plane even when the housing 7 is well offvertical. This allows the housing 7 to be mounted on the supportingtripod 25 by the operator so as to be only approximately vertical, andthus avoids the need to have a set of footscrews under the housing 7 tobring the automatic mechanism within its operating range as in othermechanisms.

Damping is required to bring the pendulum to rest in a reasonable time.This is achieved by using electro-magnetic damping. A non-magnetic metalplate 26 moving in the magnetic field of magnets 27 causes currents toflow in the plate 26 which generate a magnetic field opposing the motionof the pendulum.

The window 19 is at an angle to the vertical, tilted forwards asillustrated or rearwards to allow the undesired reflections from it tobe absorbed by the interior of the container as shown at 28. The tworays 16 (FIG. 1) represent light rays travelling from the light source12 to the observer's eye via the mirror pair 6.

In order for the pendulum 20 to come to rest with both mirrors of themirror pair exactly vertical, it is necessary to be able to alter thecenter of gravity of the pendulum. This is achieved by two adjustingscrews 46 set perpendicular to each other.

An improved advantage in using a self-levelling device in contrast tospirit levels as shown in the prior art patent referred to above is thatuneven thermal expansion of the parts of the device is much less likelyto tilt the mirrors away from vertical. In the present invention unevenexpansion of the critical parts, being the pendulum and mirror assembly,is further reduced by having them enclosed in the housing 7.

There are two modes of operation when using this embodiment of theinvention. FIG. 4 is a side elevation illustrating the usual mode ofoperation, and FIG. 5 is an end elevation looking towards the mirrorpart 6. The levelling rod 14 carries fitting 13, which can be moved upand down the rod, the fitting carrying the light source 12 and the eyepiece 15, both of which may or may not be fitted with optics. Thefitting 13 is moved up and down rod 14 until the observer sees areflection of the light in the mirror pair 6. The height of the fittingis then read off the graduation 29 on the levelling rod. If the sameprocess is repeated at the second location, the difference in elevationof the two locations is equal to the difference in the rod readings. Theeyepiece could incorporate a bubble which could be seen while looking athousing 7, similar to that used on eye levels. This would immediatelytell the observer whether to move the fitting up or down to find thecorrect level and the desired reflection.

FIG. 6 illustrates the use of the invention on sloping ground which isthe second mode of operation. In this case the eyepiece 15 is detachedfrom the fitting 13, and remains at a convenient height for theobserver, shown in FIG. 6 as 1.5 meters while the light moves up anddown the rod. Two separate locations of the staff are shown. In eachcase it can be seen that the horizontal line from the mirror pair 6 lieshalfway between the light source 12 and the eyepiece 15.

The upright decimal numbers 29 represent the ordinary set of graduationson the rod. The desired reading could be deduced in each case by readingthe positions of the light source, and the eyepiece on these graduationsand averaging them. However, this would be inconvenient. To avoid havingto average two readings, an alternate set of graduation 31, is provided.These are arranged so that the reading at the eyepiece is the same onboth the alternative and the normal graduations, and each meter on thealternative graduations is graduated to read as through it was half ameter, that is, the alternative graduations enlarged by a factor of two.

It is possible to determine the desired reading directly by reading theposition of the light source 12 on the alternative scale. It can be seenin. FIG. 6 that the reading at the light source 12 on each rod is thesame as is obtained by averaging the readings of the light and theeyepiece on the normal set of graduations.

FIGS. 7, 8, and 9 illustrate another embodiment of the invention whereinthe light reflecting device is a right angle prism 32. Prism 32 issuitably silvered or coated to provide the right-angled reflectingsurfaces.

Prism 32 is located on the upper end of a pendulum 33 suspended from theupper part of a container 34 located in the housing 7, the container 34serving to protect prism 32 and the pendulum and its suspension fromwind and dust, and the housing 7, as in the previous embodiment, has asits main function the provision of a dark background against which tosee the reflection of the light source 12 and to block out undesiredreflections from brightly illuminated objects such as wall 18.

The housing 7, in this embodiment, has an opening 35 and the innercontainer 34 has a window 36 to allow light to enter.

The pendulum 33 is suspended in container 34 from a gimbal arrangement37 which in turn supports a disc or plate 38 from which depends flexibletapes 39. At each level, one, two or more tapes, membranes or flexiblethreads or wires or the like could be used provided that they all lie inthe same vertical plane so as to allow the pendulum to be able tooscillate in a plane perpendicular to the plane of the tapes 39 or thelike.

The tapes 39 support a disc or plate 40 from which depends another setof tapes or the like 41 which support the pendulum 33 for oscillatorymovement in a plane perpendicular to the plane of the tapes 41. Thependulum 33 is thus free to oscillate as a pendulum in any plane withoutrotating about its vertical axis.

Although other pivoting arrangements cold be used for the pendulum, aparticular advantage of the arrangement shown in FIG. 7 is that itoperates effectively in any vertical plane even when the container 34 iswell off vertical. This allows the housing 7, as in the previousembodiment, to be mounted on the supporting tripod 25 by the operator soas to be only approximately vertical, and thus avoids the need to have aset of footscrews under the housing 7 to bring the automatic mechanismwithin its operating range as in other mechanisms.

Damping is achieved by using a non-magnetic metal plate 42 moving in themagnetic field of the magnets 43 which causes currents to flow in theplate 42 which generate a magnetic field opposing the motion of thependulum.

The window 36 is at an angle to the vertical titled forwards asillustrated or rearwards to allow the undesired reflections from it tobe absorbed by the interior of the housing 7 as shown at 44. The ray 45represents a light ray travelling from the light source 12 to the prism32 and back towards the source.

In order for the pendulum 33 to come to rest with the mirrored surfaceof the prism 32 exactly vertical, it is necessary to be able to alterthe center of gravity of the pendulum or alter the point of suspensionor the angle between the prism and the pendulum axis. The requiredverticality of the mirrored surfaces is achieved by two adjusting screws46 set perpendicular to each other, which alters the position of thecenter of gravity of the pendulum.

The modes of operation when using this embodiment of the invention aresimilar to those described in the previous embodiment.

FIG. 9 is an elevation of the prism 32 taken through its axis ofsymmetry. For simplicity the earlier description has been in terms ofthe prism having its two reflecting surfaces in perpendicular verticalplanes. If mirrors are used as in the previous embodiment thisassumption is correct but when a prism is used it may be necessary toeliminate undesirable reflections from the front surface 47 of the prism32.

This is achieved by having its surface sloping down as shown (or up) bya small angle. Undesirable reflections 48 from brightly lit, objects inthe field of view (such as a car windshield with the sun reflecting fromit) are absorbed by the inside of the housing 7.

Because a horizontal ray 49 will be refracted as shown by ray 50 theprism needs to be tilted so that the edge 51 where the two mirroredsurfaces meet is perpendicular to the ray 50, and hence the planes ofthe mirrored surfaces are slightly off the vertical.

FIG. 10 is a plan view of an arrangement of the invention adapted tothat it provides a field of view of 360° (provided that the observer ismore than a few meters away from it).

The outer housing 7 of this embodiment has a number of inner containers34 each with its own right angle pair of mirrors or prism and pendulumassembly, arranged so that the fields of view 52 overlap, provided thatthe observer is a few meters away.

The invention also contemplates the substitution of a source ofelectromagnetic radiation, for example infra-red radiation, for thelight source. This would allow the use of a sensor to detect the beaminstead of using the eye, which would be advantageous in somecircumstances.

Claim:
 1. A device for defining a plane to determine relative elevationscomprising in combination a stationary apertured housing part locating apair of vertical reflecting surfaces at right angles to each other and alevelling rod adapted to be positioned at a viewing distance from saidhousing part and incorporating a light source and an observation sightwhich allows the desired plane to be found by observing the reflectionof the light in said pair of reflecting surfaces.
 2. A device accordingto claim 1 wherein said light source and said observation sight aremounted on a fitting slidable on said levelling rod.
 3. A deviceaccording to claim 1 wherein said light source is mounted on a fittingslidable on said levelling rod and said observation sight is located ata fixed position on said levelling rod.
 4. A device according to claim 3wherein said pair of reflecting surfaces comprise a pair of mirrorsjoined at right angles to each other.
 5. A device according to claim 3wherein said pair of reflecting surfaces comprise a right angle prismsuitable coated to provide two reflecting surfaces at right angles toeach other.
 6. A device according to claim 1 wherein said observationsight comprises an eyepiece.
 7. A device according to claim 1 whereinsaid front surface and said rear reflecting surfaces are mounted on aself-levelling pendulum within said housing part.
 8. A device accordingto claim 7 wherein the pendulum is supported within the housing part bya system of two or more groups of tapes or threads attached betweendiscs wherein each group is located on said discs in a vertical planeperpendicular to the adjacent group.
 9. A device according to claim 7and including electromagnetic damping means located in said housing partbelow said pendulum.
 10. A device according to claim 1 wherein the planeto be defined is a horizontal plane.
 11. A device according to claim 1wherein said pair of reflecting surfaces comprises a pair of mirrorsjoined at right angles to each other.
 12. A device according to claim 1wherein said pair of reflecting surfaces comprises a coated right-angleprism to provide two reflecting surfaces at right angles to each other.13. A device for defining a desired plane to determine relativeelevations comprising in combinationa stationary apertured housing part;a right-angled prism supported in said housing part and havinga frontsurface tilted from the vertical so as to eliminate undesirablereflections from said front surface, and a pair of coated rearreflecting surfaces at right angles to each other which are tiltedslightly from the vertical by an amount selected to compensate forrefraction of a horizontal ray as it passes through the front surface ofthe prism and to allow a horizontal light ray entering the prism to bereflected in a horizontal direction upon leaving the prism; and alevelling rod adapted to be positioned at a viewing distance from saidhousing part and including a light source and an observation sight toallow the desired plane to be found by observing reflection of the lightin said pair of reflecting surfaces.
 14. A device according to claim 13wherein said front surface and said rear reflecting surfaces are mountedon a self-levelling pendulum within said housing part.